FIG. 2 shows a prior art MSM type photodiode as described in IEEE Electron Device Letters, Vol. EDL-2, No. 5 (1981), pages 112 to 114. In FIG. 2, an undoped GaAs layer 3 for absorbing light is grown on a semi-insulating GaAs substrate 4 by, for example, a metal organic chemical vapor deposition (hereinafter referred to as "MOCVD") method. A pair of Schottky electrodes 1 and 2 are disposed on the light absorption layer 3 at predetermined positions to produce Schottky junctions with the layer 3. Light is incident to the device as shown by an arrow 6.
In operation, a bias voltage is applied between the electrodes 1 and 2. Then, the Schottky barrier at electrode 2 is reverse-biased and the Schottky barrier at electrode 1 is forward biased. Therefore, a depletion layer is produced extending from the electrode 2 toward the electrode 1 having a volume that increases as the applied bias voltage is increased. In order to obtain high speed response, a sufficiently high voltage is applied so that a region between the electrodes 1 and 2 is depleted in the neighborhood of the surface of light absorption layer 3. When light 6 is incident on the biased photodiode, the incident light 6 is absorbed in the light absorption layer 3 and generates electron-hole pairs. The generated electron-hole pairs are separated by the applied electric field and electrons and holes reach the electrodes 2 and 1, respectively, and collected as a photocurrent.
Since the prior art MSM type photo detector is produced on a semi-insulating GaAs substrate 4 and both electrodes 1 and 2 are disposed on a common surface, it has a high degree of compatibility with other electronic circuits and has high speed response and low capacitance. These characteristics make the light receiving element popular in optoelectronic integrated circuits (so-called OEIC).
The prior art MSM type semiconductor light responsive element having the described construction has problems as described below because the light absorption layer 3 is exposed at the uppermost surface of the element.
FIG. 3 shows the distribution of the electric field in the cross-section of the light responsive element. As shown in the figure, the electric field is concentrated at the portions of the electrodes 1 and 2 nearest of each other. Since the generated carriers are concentrated at the nearest portions of the electrodes 1 and 2 and the electric field is strong at the nearest portions of the electrodes, largest variations in the electric field occur at the nearest portions of the electrodes 1 and 2. Furthermore, carriers generated in the light absorption layer 3 are captured by and ionizes traps in the crystal. The lifetimes of the traps vary dependent on, for example, crystallinity or surface properties of the crystal in a range of from about several milliseconds to several microseconds. When these lifetimes are sufficiently long compared with the modulation frequency of the incident signal light, the ionized traps effectively become stationary charges, changing the distribution of the electric field and the flow of the charge of carriers. When the light absorption layer 3 is exposed at the uppermost surface the element and has a large number of surface states, a large number of carriers are trapped, increasing the carrier flow phenomenon further.
FIG. 4(b) shows the energy band structure between the electrodes in the neighborhood of semiconductor surface. When no light is incident on the element, the conduction band edge 8 and valence band edge 9 are positioned at the potentials shown in the figure by the solid lines. When light is incident on the element and carriers are generated at the light absorption layer 12, however, the carriers generated as discussed above are captured by the traps in the neighborhood of the surface of light absorption layer 12 and the traps are ionized and become stationary charges. Therefore, the potentials of the conduction band edge and valence band edge shown by broken lines 8' and 9' of figure 4(b) are lowered and the slope of the bands at the uppermost surface of the light absorption layer 12 becomes steeper. As a result, the width of the barrier through which electrons may tunnel is narrowed, increasing the probability of tunneling and the injection of majority carriers. That phenomenon produces an unstable current amplification and deteriorates the frequency response of the photodiode. In FIG. 4(b), reference numeral 10 designates the Fermi level position in layer 3 and reference numeral 11 designates the position of the Schottky barrier.